The preferred embodiment relates to a limit signal indicator or limit signal transmitter for determining a position such as an end position of a pneumatically operated positioning device. Limit signal indicators are often applied for safety closure valves, which can be operated pneumatically. In case of an operational malfunctioning the pneumatic power source is cut off, whereby the safety closure valve automatically, by means of stored spring energy, reaches a final safety position in which a fluid transmission line of a technical processing plant is either completely opened or completely closed by means of a valve member.
A common limit signal indicator should be capable of displaying to an operating person by means of a respective signal whether the safety signal valve is indeed in the desired end position.
For generating and transmitting signal information, a signal transmission contact is formed in the known limit signal indicator generally referred to as a Namur-sensor. A Namur-sensor is defined among others by the standard “IEC69047-5-6”. If the positioning device adopts the expected end position, this adoption of position is represented via the signal transmission contact by a current signal of less than about 1.2 mA, for example about 1 mA. In case of non-adoption of the end position another specific current signal of above about 2.1 mA is given by the Namur-sensor. In order to generate these two binary switching states, a buffer amplifier can be connected to the Namur-sensor, wherein the buffer amplifier can be connected to a so-called proximity switch. The proximity switch is supplied with about 8 V and modifies the supply current to the buffer amplifier depending on the switch status according to the above named amounts, that is between the limit values 1.2 mA and 2.1 mA. If the input current for the buffer amplifier drops below a value of about 0.1 mA or if no current is supplied to the buffer amplifier, according to the standard this will be interpreted as a line breakage towards the Namur-sensor. According to the standard EN 60947-5-6 the buffer amplifier will then generate a standardized emergency signal of below 0.1 mA. If however, the buffer amplifier generates the standardized emergency signal with a current value of more than about 6 mA, a short circuit at the limit signal indicator is to be assumed. The two above-named switching states (1.2 mA; 2.1 mA) are displayed and transmitted by the buffer amplifier as standardized switch signals, wherein also two additional emergency signals, that is a short circuit signal and a line breakage signal, can be generated and transmitted.
There is a general interest in designing limit signal indicators more “intelligently” by providing the limit signal indicator with a microprocessor and a position sensor while maintaining the basic functionality as limit signal indicator.
DE 10 2006 049 651 A1 discloses such an intelligent position sensor in which the microprocessor is electrically supplied either by the above named Namur-sensor supply voltage of 7 to 8 V or by a constant voltage signal of 24 V, commonly supplying a pneumatic valve that in turn is connected to a pneumatic positioning drive. Another intelligent limit switch arrangement is known by EP 1 730 611 A1.
However, the increase of functional multiplicity of the limit signal indicator leads to the problem that in view of the restrictive safety regulations for technical processing plants, the functionality of the intelligent limit signal indicators must constantly be surveyed and malfunctioning must as far as possible be excluded.